Compatibility between PAN-based carbon fibres and Mg8Li alloy during the pressure infiltration process

The interaction of PAN-based T800H type carbon fibres, which had been coated with (i) pyrocarbon and (ii) pyrocarbon+SiC protective layers, was studied by means of SEM, energy-dispersive X-ray analysis and Auger electron spectroscopy. The interaction occurred during the pressure infiltration process at temperatures from 898 to 908 K and at contact times of 4–30 s. As detected by AES measurements, both Mg and Li penetrated throughout the carbon fibres, and Li₂C₂ was formed without remarkable reaction zone occurrence, which led to carbon fibre degradation. The pyrocarbon layer (about 50 nm thick) demonstrated a good protective efficiency and the corresponding metal-matrix composite exhibited satisfying strength characteristics; nevertheless, the influence of processing variables (temperature, time) was obvious. On the other hand, the double-coated pyrocarbon+SiC carbon fibres were strongly affected by Li₂C₂ formation.     

Nickel- and copper-coated carbon fibre reinforced tin-lead alloy composites

Nickel and copper were deposited over brominated, surface treated, and pristine P-100 carbon fibres using cementation and electroplating techniques. The fibres were brominated by bromine vapour for 48 h and then desorbed at 200 °C in air for 12 h. The anodic oxidation treatment of the fibres involved electrochemical etching in a dilute sodium hydroxide electrolyte for 3 min. Electroplated coated fibres showed better tensile properties than cementation coated fibres. In addition, nickel coating exhibited better bonding with the carbon fibres compared to copper coating. The effect of bromination and surface treatment was improved adhesion between coating and fibres. Nickel- and copper-coated fibres, which were brominated, anodically oxidized, and pristine, reinforced tin-lead alloy composites were fabricated by squeeze casting. The composites containing coated treated carbon fibres had higher tensile and shear strength than the ones containing coated pristine carbon fibres. Moreover, the composite with coated brominated carbon fibres had better tensile strength and shear strength than the surface treatment. The results also showed the composites containing nickel-coated fibres had higher tensile and shear strength than the ones containing copper-coated fibres.     

碳纤维涂覆SiC新工艺及涂层纤维的力学特性

用气相SiO和CO反应形成SiC的方法成功地在碳纤维上涂覆SiC.SiC涂层是由β-SiC团粒从碳纤维表面向外生长形成,并具有玉米棒子状的表面形貌。SiC层厚度增加到≥0.2μm,涂层纤维的拉伸强度随SiC层厚度增加而下降,此结果与Ochiai的理论模型相吻合。SiC层厚度达到1μm的涂层纤维,其拉伸时的力学行为与具有弱界面结合的1维脆性纤维-脆性基质复合材料相似,并显示SiC层的多次断裂。对碳纤维芯因涂层处理而引起的性能退化进行了讨论。      

Interfacial reaction and strength of SiC fibres coated with aluminium alloys

Silicon carbide fibres (Nicalon) were coated with pure aluminium and aluminium alloys containing silicon. The coated fibres were annealed to produce an interfacial reaction zone between the coated layer and the fibre. The effect of this reaction zone on the tensile strength of the fibre was investigated. During the early stages of growth the reaction zone of the fibre is thin, and the strength of the fibre is controlled by inherent defects so that the fibre retains its original strength. After the early stages, notches are formed in the reaction zone of the fibre on loading at a small strain and the fibre fractures when a notch extends into the fibre. In this stage the fibre strength is dependent on the thickness of the reaction zone. An alloying addition of 1 or 5 at % Si to the aluminium matrix was found to be effective in retarding the growth rate of the reaction zone.On leave from Institute of Metal Research, Academia Sinica, Shenyang, China.On leave from Instituto Superior Tecnico, Departamento de Metalugia e Materais, Lisbon, Portugal.On leave from Hitachi Research Laboratory, Hitachi Ltd, Saiwai-Cho, Hitachi, Ibaraki 317, Japan.     

Effects of HF treatments on tensile strength of Hi-Nicalon fibres

Tensile strengths of as-received Hi-Nicalon fibres and those having a dual BN–SiC surface coating, deposited by chemical vapour deposition, have been measured at room temperature. These fibres were also treated with HF for 24 h followed by tensile strength measurements. Strengths of uncoated and BN–SiC coated Hi-Nicalon fibres extracted from celsian matrix composites, by dissolving away the matrix in HF for 24 h, were also determined. The average tensile strength of uncoated Hi-Nicalon was 3.19±0.73 GPa with a Weibull modulus of 5.41. The Hi-Nicalon–BN–SiC fibres showed an average strength of 3.04±0.53 GPa and Weibull modulus of 6.66. After HF treatment, the average strengths of the uncoated and BN–SiC coated Hi-Nicalon fibres were 2.69±0.67 and 2.80±0.53 GPa and the Weibull moduli were 4.93 and 5.96, respectively. The BN–SiC coated fibres extracted from the celsian matrix composite exhibited a strength of 2.38±0.40 GPa and a Weibull modulus of 7.15. The strength of the uncoated Hi-Nicalon fibres in the composite was so severely degraded that they disintegrated into small fragments during extraction with HF. The uncoated fibres probably undergo mechanical surface damage during hot pressing of the composites. Also, the BN layer on the coated fibres acts as a compliant layer, which protects the fibres from mechanical damage during composite processing. The elemental composition and thickness of the fibre coatings were determined using scanning Auger analysis. Microstructural analyses of the fibres and the coatings were done by scanning electron microscopy and transmission electron microscopy. Stengths of fibres calculated using average and measured fibre diameters were in good agreement. Thus, the strengths of fibres can be evaluated using an average fibre diameter instead of the measured diameter of each filament. © 1998 Kluwer Academic Publishers     

Fibre-reinforced glasses: influence of thermal expansion of the glass matrix on strength and fracture toughness of the composites

The reinforcement of glasses by incorporation of fibres was considered to depend on the force transfer from the matrix on the fibres in order to obtain optimum strength and fracture toughness. This may occur by thermal shrinking of the matrix on the fibres after the hot-pressing procedure. It is shown that an optimum exists for strain and stress transfer from the matrix to the fibres if this shrinkage process is neither so strong that no pull-out and no bend-over effect is produced nor so weak that no stress transfer is possible. Therefore, experiments were performed with Nicalon-SiC fibres and with selected glasses which show different thermal expansion coefficients. In this way it was possible to produce fibre-reinforced glass composites with well-tailored special properties. Estimations of tensile stresses within the glass matrix led to values which are partly above those of the bulk glass. Because no cracks occurred during cooling and during heat shock treatment fromT _g, it was concluded that the strength of the thin glass layers between the very smooth surfaces of the Nicalon-SiC fibres cannot be compared with that of bulk glass but with that of protected (coated) glass fibres or thin sheet glass.     

The effect of service-temperature exposure on the tensile properties of Ti-1421/SiC metal matrix composites

Composites consisting of a Ti-14Al-21Nb (wt%) aluminide matrix and SCS-6 SiC fibres were fabricated by plasma-spraying and hot-isostatic-pressing. The room temperature longitudinal tensile properties were evaluated after processing and after isothermal and cyclic exposure in air and in argon. The isothermal ages were performed at 760°C, and the thermal cycle was from room temperature to 760°C, which is the projected service temperature for Ti-aluminide-based composites. Some loss of tensile strength occurred after isothermal and cyclic exposure for long times (8 to 12 weeks) in argon, with no apparent difference in behaviour between the isothermal and cyclic samples. Substantial degradation in tensile properties was seen after isothermal exposure in air, while a much more severe loss in properties occurred after cycling in air. In the isothermal samples exposed in air, degradation in properties was associated with surface embrittlement of the matrix, with the longitudinal room temperature tensile strength decreasing linearly with increasing thickness of the embrittled zone. During air-cycling, this effect was enhanced by a rapid decrease in the strength of the SCS-6 fibre, and an increase in the amount of fibre breakage. Isolating the fibres from direct contact with the environment resulted in a decrease in the amount of composite strength degradation after air-cycling. This revised version was published online in November 2006 with corrections to the Cover Date.     

Studies on nickel coated carbon fibres and their composites

Uniform and continuous coating of nickel was given to the carbon fibres by cementation, electroless or electroplating techniques. The coating thickness was ranged between 0.2 and 0.6 m for all the three methods used. Coating thickness less than 0.2 m showed discontinuous coating of nickel over the fibre surface. Beyond 0.6 m thickness, nickel deposited in den-drite form over the continuous coating. For continuously coated fibres, the ultimate tensile properties of electroless coated fibres were near to uncoated carbon fibres suggesting adherent and defect free coating; while fibres coated by electrolytic and cementation process exhibited lower ultimate tensile strength (UTS) properties. The tensile fracture of the cementation coated fibres suggested degradation of the fibres. In composites, prepared by dispersing the coated fibres in pure aluminium matrix, no appreciable fibre-metal interaction was observed. NiAl₃ intermetallics were observed around and adjacent to the carbon fibres. Sometimes carbon fibres were found embedded in massive NiAl₃ intermetallics suggesting that fibre surface can also act as nucleating centre for these precipitates.     

Short sisal fibre reinforced bacterial cellulose polylactide nanocomposites using hairy sisal fibres as reinforcement

“Hairy” bacterial cellulose coated sisal fibres were created using a simple slurry dipping process. Neat sisal fibres were coated with BC to create (i) a dense BC coating around the fibres or (ii) “hairy” fibres with BC oriented perpendicular to the fibre surface. These fibres were used to produce hierarchical sisal fibre reinforced BC polylactide (PLLA) nanocomposites. The specific surface area of the BC coated fibres increased when compared to neat sisal. Single fibre tensile tests revealed no significant difference in the tensile modulus and tensile strength of “hairy fibres”. However, when sisal fibres were coated with a dense BC layer, the mechanical fibre properties decreased. The tensile, flexural and visco-elastic properties of the hierarchical PLLA nanocomposites reinforced by both types of BC coated sisal fibres showed significant improvements over neat PLLA.     

The degradation in tensile strength of polymer-coated glass optical fibres under γ-irradiation

The tensile strength of glass optical fibres when coated with various polymers has been measured as a function of -ray dose. Fibres protected with acrylate, silicone + acrylate or polyimide coatings showed little degradation after receiving a total dose of 1 MGy (they retained >95% of their preirradiated strength). For a fibre with an extruded nylon overcoat the nylon became very brittle after 0.5 MGy, but as far as could be assessed, the strength of the central glass fibre was little affected. Two other types of fibre, both protected with fluorinated polymers, were severely weakened after 0.1 MGy (their tensile strength being reduced to < 40% of their pre-irradiated strength). Experimental results are given supporting the hypothesis that the degradation results from gaseous fluorine-containing species chemically attacking the surface of the glass fibre.     

Silicon carbide (NicalonTM) fibre-reinforced borosilicate glass composites: mechanical properties

The objective of this study was to assess the applicability of an extrinsic carbon coating to tailor the interface in a unidirectional NicalonTM–borosilicate glass composite for maximum strength. Three unidirectional NicalonTM fibre-reinforced borosilicate glass composites were fabricated with different interfaces by using (1) uncoated (2) 25 nm thick carbon-coated and (3) 140 nm thick carbon coated Nicalon fibres. The tensile behaviours of the three systems differed significantly. Damage developments during tensile loading were recorded by a replica technique. Fibre–matrix interfacial frictional stresses were measured. A shear lag model was used to quantitatively relate the interfacial properties, damage and elastic modulus. Tensile specimen design was varied to obtain desirable failure mode. Tensile strengths of NicalonTM fibres in all three types of composites were measured by the fracture mirror method. Weibull analysis of the fibre strength data was performed. Fibre strength data obtained from the fracture mirror method were compared with strength data obtained by single fibre tensile testing of as-received fibres and fibres extracted from the composites. The fibre strength data were used in various composite strength models to predict strengths. Nicalon–borosilicate glass composites with ultimate tensile strength values as high as 585 MPa were produced using extrinsic carbon coatings on the fibres. Fibre strength measurements indicated fibre strength degradation during processing. Fracture mirror analysis gave higher fibre strengths than extracted single fibre tensile testing for all three types of composites. The fibre bundle model gave reasonable composite ultimate tensile strength predictions using fracture mirror based fibre strength data. Characterization and analysis suggest that the full reinforcing potential of the fibres was not realized and the composite strength can be further increased by optimizing the fibre coating thickness and processing parameters. The use of microcrack density measurements, indentation–frictional stress measurements and shear lag modelling have been demonstrated for assessing whether the full reinforcing and toughening potential of the fibres has been realized. This revised version was published online in November 2006 with corrections to the Cover Date.     

The cementation technique for coating carbon fibres

A tow of carbon fibres was coated with copper, nickel and cobalt by the cementation process. Conditions were optimized to obtain continuous, uniform and adherent coatings on three types of carbon fibres. The surface of the carbon fibres was activated by heating in vacuum at 700° C for about 15 min. The fibres were suspended in an aqueous solution of metal (Cu, Ni, Co) salt which also contained glacial acetic acid. To the solution a displacing agent such as Mg, Al, Zn or Fe was added. The metal was displaced from the solution and plated onto carbon. The thickness of the coating was dependent on the metal salt solution concentration and concentration of the surface activator (glacial acetic acid) in the solution. The appearance (dullness or brightness) of the metal coating was dependent on the concentration of the activator and the amount of displacer metal added. Mechanical testing of the single fibres indicated that the strength and modulus of the coated fibres was dependent on the coating thickness. In general, an increase in the coating thickness decreased the mechanical properties of the coated fibres. The coated fibres were introduced into an aluminium matrix successfully by the vacuum-infiltration technique to obtain composites with a uniform distribution of fibres.     

Sizing resin structure and interphase formation in carbon fibre composites

The single-embedded filament fragmentation test has been used to study the effect of fibre coatings on the adhesion of surface treated (oxidized) Type A and HS carbon fibres to an epoxy matrix. The presence of a sizing resin on the as-received fibres reduced the interfacial shear strength of the composite. For the unsized fibres, which were coated in the laboratory from commercial aqueous based sizing emulsions, a molecular weight dependence was observed. This suggests that compatibility of the deposited size with the matrix determines the adhesive bond between fibre and matrix and the formation of an interphasal region. On the other hand, deposition of a sizing resin from solution led to the differing conclusion that chemical interaction with the fibre surface had occurred. During composite fabrication these sizing resins will therefore have to act as ‘coupling agents’ to the matrix. Solvent extraction of emulsion-deposited sizing resins, particularly at elevated temperatures, appeared to promote their interaction with the fibre surface. The same trends in interfacial shear strength were observed in a second epoxy resin matrix of higher modulus, albeit at an increased magnitude. In this way, the plasticizing role of the ‘low’ molecular weight emulsion based size could be identified. Maximum likelihood statistics have been used to estimate the standard deviation on the value of interfacial shear strength.     

Resorbable continuous-fibre reinforced polymers for osteosynthesis

Four institutes from three countries in the European Union have collaborated under the BRITE–EURAM framework programme for the development of processing technologies for resorbable osteosynthesis devices. The devices should be continuous-fibre reinforced, and the technology should offer the possibility of orienting the fibres in the main trajectories. Poly-L-lactide and poly-L–DL-lactides have been synthesized for reinforcement fibres and matrix material, respectively. Melt-spun P-L-LA fibres of a strength of 800 MPa have been embedded in an amorphous P-L–DL-LA 70 : 30 matrix by compression moulding. Ethyleneoxide sterilized samples have been tested in vitro and in vivo. A satisfying bending modulus has been reached (6 GPa). Yet with 50% strength retention after ten weeks, fast degradation occurred that could be related to residual monomers. By this fast degradation 70% resorption after one year could be observed in the non-functional animal studies in rabbits. There was only a mild inflammatory reaction, which confirmed the good biocompatibility of the materials even during the resorption period. Further effort has to concentrate on the reduction of initial monomer content. The great advantage of the processing method to orient fibres in the device will be utilized in prototype samples, e.g. an osteosynthesis plate with fixation holes. © 1998 Chapman & Hall     

Thermal stability of SiC fibres (Nicalon®)

The degradation behaviour of Nippon Carbon Co. SiC fibres (Nicalon®) after heat treatment in various environments was studied. Regardless of the heat-treatment conditions, the Nicalon® fibre strength degraded when the fibres were subjected to temperatures higher than 1200° C (temperatures below 1200° C were not investigated). This degradation is associated with the evaporation of CO from the fibres as well as with-SiC grain growth in the fibres.     

Silver coating on carbon and SiC fibres

Electroless silver coating on carbon fibres using silver nitrate solutions has been studied. It was observed that the rate of silver coating depends on the degree of graphitization of carbon fibres. Fibres with a higher degree of graphitization were coated faster than those with a lower degree of graphitization. A physical model considering the number of nucleation sites on the carbon fibre surface as a function of the degree of graphitization is proposed for the silver coating process. The strength and modulus of coated and uncoated fibres have been determined using a high-sensitivity load cell with an Instron tensile testing machine. It was observed that silver coating did not alter the strength or modulus of the fibre. Aluminium matrix composites have been successfully fabricated with these fibres. The same coating technique was also used to coat silicon carbide fibres. Improvement in the infiltration during composite fabrication was observed when the fibres were silver-coated.     

The role of interfaces and matrix void nucleation mechanism on the ductile fracture process of discontinuous fibre-reinforced composites

The role of fibre morphology, interface failure and void nucleation mechanisms within the matrix on the deformation and fracture behaviour of discontinuous fibre-reinforced composites was numerically investigated. The matrix was modelled using a constitutive relationship that accounts for strength degradation resulting from the nucleation and growth of voids. For the matrix, two materials exhibiting identical strength and ductility but having different void-nucleation mechanisms (stress-controlled and strain-controlled) were considered and fibres were assumed to be elastic. The debonding behaviour at the fibre interfaces was simulated in terms of a cohesive zone model which describes the decohesion by both normal and tangential separation. The results indicate that in the absence of interface failure, for a given fibre morphology the void nucleation in the matrix is the key controlling parameter of the composite strength and ductility, hence, of the fracture toughness. The weak interfacial behaviour between the fibres and the matrix can significantly increase the ductility without sacrificing strength for certain fibre morphology and for certain matrix void-nucleation mechanisms.     

Effect of carbon monoxide partial pressure on the high-temperature decomposition of Nicalon fibre

The high-temperature equilibrium partial pressures of the predominant gaseous species over Nicalon were determined thermochemically. It was calculated that the most prevalent gaseous species in equilibrium with Nicalon at 1300 °C is carbon monoxide. Subsequently, fibres of Nicalon (NLM 202) were heat treated at 1300 °C in various partial pressures of carbon monoxide gas and analysed via single filament strength testing, scanning electron microscopy, X-ray diffraction, and scanning Auger microscopy. The heat treatments in carbon monoxide had a significant effect on the strength retention and composition of the fibres (75% retained) compared to the treatments in argon where only 25% of the initial strength was retained. The Auger analysis revealed that the treatment in argon evolved carbon and oxygen from the fibre while in carbon monoxide atmospheres a carbon layer was deposited on the fibre surface. X-ray diffraction showed that grain growth had not occurred in any of the heat treatments. This study shows the important role of thermochemical reactions in the strength degradation of Nicalon, and its possible relationship to the formation of carbon surface/interface layers.     

Compatibility between carbon fibre and binary aluminium alloys

Compatibility between carbon fibre and various binary aluminium alloys is investigated. Series of aluminium alloys are coated onto the fibre surface and the wetting behaviour of these alloys are observed by scanning electron microscopy after heating the coated fibres at 1073 K, well above the melting point of the alloys. It is found that the aluminium alloys containing such elements as indium, lead and thallium showed excellent wetting behaviour. These alloying elements have significantly smaller surface energies as compared to aluminium and they are practically insoluble with aluminium, even in the liquid states. Tensile test shows that the strength of carbon fibres is not degraded after heating at 1073 K when fibres are coated with an Al-1 at % Tl alloy. The reasons for these elements in aluminium to substantially improve the compatibility against carbon fibres are discussed from the thermodynamical point of view.     

Effect of silane coupling agent on mechanical performance of glass fibre

Mechanical performance of commercially manufactured unsized and γ-aminopropylsilane sized boron-free E-glass fibres has been characterised using single-fibre tensile test. Both apparent fibre modulus and fibre strength were found to strongly depend on fibre gauge length. The average strength of sized fibres was found 40–80 % higher than unsized fibres at different gauge lengths. Weibull analysis suggested that the failure mode of unsized fibres could be described by unimodal Weibull distribution, whereas the strength distribution of sized fibres appeared to be controlled by two exclusive types of flaw population, types A and B. Comparison of the Weibull plots between unsized and sized fibres revealed that the strength of unsized fibres was likely to be dominated by type A flaws existing on the bare glass surface and type B flaws may be related to the defects on the glass surface coated with silane. This was partially supported by the observation of fractured cross-sectional area using SEM. It was, therefore, proposed that the strength difference between unsized and sized glass fibres may be more reasonably interpreted from the surface protection standpoint as opposed to the flaw healing effect. The results obtained from this study showed that silane coupling agent plays a critical role in the strength retention of commercially manufactured E-glass fibres and the silane effect on the fibre strength is also affected by the change in gauge length of the sample.